Nowadays, emails are rarely a substitute for a normal letter. They are mainly used to exchange information. Not only does reading emails take a long time, writing them does, too. That is why it makes sense to focus on what is relevant. We live in a time where time is money, so time is our most valuable asset. Not only for us as email senders, but also for those on the receiving end.
If you regularly write emails with long-standing clients or colleagues, it makes sense to abide by the five-sentence rule. This means that you are committed to keeping emails as short as possible: as much information as necessary in as few sentences as possible.
The time saved can then be spent, for example, having a coffee with your most favorite client. :)
Just make sure that your reader knows that you are applying this 5-sentence rule in your email communication. You can, for instance, include this in your email signature:
Q: Why is this email five sentences or less?